For many years, portable film and video cameras have been available for private or “home movie” use. Focusing may be achieved by auto focusing through the lens, or by aligning a focus distance number for example located on a focus ring on the lens with a focus index mark. While such arrangements are light and highly portable, they are satisfactory only for low end applications. At the other extreme, the highest performance camera equipment has been employed in professional cinematography for taking professional motion pictures. In this application, a camera operator will typically control the composition of the shot by panning and tilting the camera, and optionally zooming, while a camera assistant, called a “focus puller” will attend to ensuring that the camera is in focus during the shot. This may be achieved by rotating a focus ring on the lens in order to align a focus distance mark on a focus scale located on the lens with a focus index mark normally fixedly located on the lens. Alternatively, a “follow focus” motor drive may be employed for altering the focusing of the lens. In such lenses, the focus scale will typically not be simply an array of focus distance numbers as in the cases of amateur equipment with the consequential error in setting the focus distance caused by the finite dimensions of the numbers, but will instead be an array of focus distance marks of small lateral dimensions, each mark being associated with a number giving the focus distance in feet or meters. These focus distance marks may be represented by lines nearby the focus distance numbers. Alignment of focus distance marks with a focus index mark will lead to a significantly more accurate setting of the focus distance. The scales will normally be calibrated before shipping, but even so, in some cases it may be desired to place blank focus distance rings on the lens after manufacture or after renting the lens and to calibrate the lens by focusing on objects at different distances and drawing marks on the ring corresponding to the distance of the object.
More recently, compact film and video camera systems which may be used in high performance imaging applications have become available. Although usually not as small as home movie cameras, such equipment may for example be employed in film making and also in “electronic news gathering” (ENG) applications in which perhaps only one operator is employed both to take the pictures and to focus the camera, and the equipment is carried on the shoulder of the operator. This has led to a demand for compact and lightweight objective lenses of the same general performance as has been customary for professional movies.
In the case of systems employing interchangeable lenses, it is desirable to improve the ease with which the lenses may be changed and focused when focusing is performed manually or by means of motors. However this may be difficult due to a number of reasons, for example with compact lens systems the circumference of the lens may be relatively small with the result that for certain focus distances the focus distance marks may become compressed.